The breathing gas temperature at the outlet of the mixing chamber with the humidified breathing gas is measured in prior-art respiration humidifiers by means of a temperature-dependent resistor element, and the heat output of the water reservoir is regulated accordingly. In disposable flexible breathing tube systems, a reusable temperature sensor, which is connected by a cable to the humidifier proper having a heating means, is plugged for this purpose into the flexible tube. To avoid cross infections, this reusable temperature sensor must be cleaned and sterilized or disinfected after use. There remains a hygienic risk if the procedure is not carried out carefully. The temperature sensor must be sufficiently sealed in the flexible tube system to prevent leaks from developing. The handling of this prior-art design or system is complicated. As an alternative, the temperature sensor may be integrated in the flexible breathing tube system as a fixed component of this system, which leads to higher costs in case of disposable flexible tube systems if the temperature sensor is disposed of together with the flexible tube system after use. Cables must be additionally led from the respiration humidifier to the flexible tube system in case of flexible tube systems used several times, which may lead to additional costs, and due to the repeated cleaning, disinfection or sterilization operations, to limited reliability of operation of the cables and electric plug connections.